SU-E-T-382: Influence of Compton Currents On Profile Measurements in Small- Volume Ion Chambers

Tanny, S; Parsai, E; Holmes, S

doi:10.1118/1.4888715

Title: SU-E-T-382: Influence of Compton Currents On Profile Measurements in Small- Volume Ion Chambers

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Abstract

Purpose: Ionization chambers in electron radiation fields are known to exhibit polarity effects due to Compton currents. Previously we have presented a unique manifestation of this effect observed with a microionization chamber. We have expanded that investigation to include three micro-ionization chambers commonly used in radiation therapy. The purpose of this project is to determine what factors influence this polarity effect for micro-chambers and how it might be mitigated. Methods: Three chambers were utilized: a PTW 31016, an Exradin A-16, and an Exradin A- 26. Beam profile scans were obtained on a Varian TrueBeam linear accelerator in combination with a Wellhofer water phantom for 6, 9, and 12 MeV electrons. Profiles were obtained parallel and perpendicular to the chamber's long axis, with both positive and negative collecting bias. Profiles were obtained with various chamber components shielded by 5 mm of Pb at 6 MeV to determine their relative contributions to this polarity effect. Results: The polarity effect was observed for all three chambers, and the ratio of the polarity effect for the Exradin chambers is proportional to the ratio of chamber volumes. Shielding the stem of both Exradin chambers diminished, but did not remove the polarity effect. However, they demonstratedmore »« less

Authors:

Tanny, S; Parsai, E ^[1]; Holmes, S ^[2]

University of Toledo Medical Center, Toledo, OH (United States)
Standard Imaging, Inc., Middleton, WI (United States)

Publication Date:: Sun Jun 01 00:00:00 EDT 2014

OSTI Identifier:: 22355926

Resource Type:: Journal Article

Journal Name:: Medical Physics

Additional Journal Information:: Journal Volume: 41; Journal Issue: 6; Other Information: (c) 2014 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0094-2405

Country of Publication:: United States

Language:: English

Subject:: 07 ISOTOPES AND RADIATION SOURCES; CABLES; IONIZATION CHAMBERS; LINEAR ACCELERATORS; PHANTOMS; RADIOTHERAPY; SHIELDING

Citation Formats


                    Tanny, S, Parsai, E, and Holmes, S. SU-E-T-382: Influence of Compton Currents On Profile Measurements in Small- Volume Ion Chambers.  United States: N. p., 2014. 
        Web.  doi:10.1118/1.4888715.

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                    Tanny, S, Parsai, E, & Holmes, S. SU-E-T-382: Influence of Compton Currents On Profile Measurements in Small- Volume Ion Chambers.  United States.  https://doi.org/10.1118/1.4888715

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                    Tanny, S, Parsai, E, and Holmes, S. 2014.  
        "SU-E-T-382: Influence of Compton Currents On Profile Measurements in Small- Volume Ion Chambers".  United States.  https://doi.org/10.1118/1.4888715.

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@article{osti_22355926,

  title        = {SU-E-T-382: Influence of Compton Currents On Profile Measurements in Small- Volume Ion Chambers},

  author       = {Tanny, S and Parsai, E and Holmes, S},

  abstractNote = {Purpose: Ionization chambers in electron radiation fields are known to exhibit polarity effects due to Compton currents. Previously we have presented a unique manifestation of this effect observed with a microionization chamber. We have expanded that investigation to include three micro-ionization chambers commonly used in radiation therapy. The purpose of this project is to determine what factors influence this polarity effect for micro-chambers and how it might be mitigated. Methods: Three chambers were utilized: a PTW 31016, an Exradin A-16, and an Exradin A- 26. Beam profile scans were obtained on a Varian TrueBeam linear accelerator in combination with a Wellhofer water phantom for 6, 9, and 12 MeV electrons. Profiles were obtained parallel and perpendicular to the chamber's long axis, with both positive and negative collecting bias. Profiles were obtained with various chamber components shielded by 5 mm of Pb at 6 MeV to determine their relative contributions to this polarity effect. Results: The polarity effect was observed for all three chambers, and the ratio of the polarity effect for the Exradin chambers is proportional to the ratio of chamber volumes. Shielding the stem of both Exradin chambers diminished, but did not remove the polarity effect. However, they demonstrated no out-of-field effect when the cable was shielded with Pb. The PTW chamber demonstrated a significantly reduced polarity effect without any shielding despite its comparable volume with the A-26. Conclusions: The sensitive volume of these micro-chambers is relatively insensitive to collecting polarity. However, charge deposition within the cable can dramatically alter measured ionization profiles. This is demonstrated by the removal of the out-of-field ionization when the cable is shielded for the Exradin chambers. We strongly recommend analyzing any polarity dependence for small-volume chambers used in characterization of electron fields.},

  doi          = {10.1118/1.4888715},

  url          = {https://www.osti.gov/biblio/22355926},
  journal      = {Medical Physics},

  issn         = {0094-2405},

  number       = 6,

  volume       = 41,

  place        = {United States},

  year         = {Sun Jun 01 00:00:00 EDT 2014},

  month        = {Sun Jun 01 00:00:00 EDT 2014}

}

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